This invention relates to an improvement in a process for making hydrogen cyanide (HCN).
HCN can be produced by a process comprising contacting in vapor phase, ammonia, a hydrocarbon, e.g., methane or natural gas, and an oxygen-containing gas in the presence of a platinum-group metal catalyst at high temperature. In commercial operations, the reactants are generally ammonia, natural gas and air. This one-stage synthesis of HCN from ammonia and natural gas in which heat is supplied by simultaneous combustion reactions with air is disclosed in U.S. Pat. No. 1,934,838 issued to Andrussow on Nov. 14, 1933. Numerous modifications and improvements pertaining to this process have been described and patented.
It is known that in the reaction of ammonia, natural gas and air to produce HCN, one of the most important variables with respect to HCN yield and conversion is the composition of the feed gas. While any two ratios of reactants can fix the feed gas composition, various combinations of these ratios can and have been employed. Optimum ratios vary depending upon operating conditions, such as throughput, catalyst type and age, preheat, and reactor geometry. Hence, the optimum ratio needs to be determined and regulated periodically so as to assure maximum productivity and yield.
Various process techniques have been developed for obtaining improved HCN yields. For example, several patents, e.g., U.S. Pat. No. 3,104,945 and British Pat. No. 956,200, describe preheating the feed gases and maintaining specified ratios of reactants. With the use of preheat it is possible to add more ammonia to the reaction without reducing the reaction or catalyst bed temperature, thus increasing production of HCN.
U.S. Pat. No. 3,370,919 issued to Pan on Feb. 27, 1968 discloses that control of the reaction of methane, ammonia and air to produce HCN is effected by measuring the reaction or flame temperature and/or off-gas temperature produced at a given air/(CH.sub.4 + NH.sub.3) ratio and adjusting the CH.sub.4 /NH.sub.3 ratio until a minimum reaction or flame temperature and/or a minimum off-gas temperature is obtained. Yield of HCN is said to be maximized at the minimum reaction or flame temperature while conversion of HCN is said to maximize at the minimum off-gas temperature. Additional details on this process can be found in an article by B. Y. K. Pan and R. G. Roth entitled "Optimization of Yield Through Feed Composition," I & EC Process Design and Development, Vol. 7, No. 1, pages 53-61 (January 1968). A further article on maximizing yield in the HCN process authored by B. Y. K. Pan and entitled "Elaboration and Extension of Experimental Results Through Mathematical correlations and Fundamental Knowledge" can be found in I & EC Process Design and Development, Vol. 8, NO. 2, pages 262-266 (April 1969).
In practice the process disclosed by Pan has not provided optimum HCN production in all Andrussow-type processes. In some such processes, controlling the reaction at minimum reaction or flame temperature has not resulted in optimum HCN production; increased production of HCN can be obtained at temperatures other than minimum bed temperature. Furthermore, in many HCN converters the reaction or flame temperature will vary considerably across the gauze catalyst, the uneven heating thus creating problems in the use of the Pan technique.